Phase locked loops are commonly used to recover data bit timing or frequency information from a noise corrupted signal in order to subsequently recover data from the signal. The recovery of data bit timing of a pulse amplitude modulated signal presents unique problems in that such signal contains both digital and analog characteristics that make utilization of standard digital techniques for clock recovery inadequate.
Pulse amplitude modulation is essentially a sampled-data type of encoding in which information is encoded into the amplitude of a train of pulses. The amplitudes of the pulses constitute the transmitted data. The pulse train itself may be considered the data carrier. Pulse amplitude modulation (PAM) is chiefly used for time division multiplex systems employing a number of channels. Usually, information is transmitted in frames of a plurality of pulses, and information is usually repeated once per frame. Each time slot or data pulse represents an analog voltage of data for a specific channel. Since each data channel is independent of each other data channel, the transition from one data pulse to the next is an indeterminate quantity. There may be a large amplitude variation between adjacent data pulses, or there may be no variation. The transitions between data bits or pulses do not necessarily ever cross zero except perhaps during the specific intervals normally used for frame synchronization, amplitude restoration and D. C. restoration.
The indeterminate nature of transitions in a PAM signal renders it difficult to use the transition between data bits to recover data bit timing. In one prior art device, a differentiator is used to detect the transitions from one bit to the next, and an absolute value circuit is used to make all transitions appear to be of the same polarity. The output of the absolute value circuit is a train of pulses having variable amplitudes that is used to generate an error voltage. The error voltage is used as the control signal in a phase locked loop to recover data bit timing.
The principal difficulty with this prior art device is implementation of the differentiator. Such differentiator must be designed to handle widely varying amplitudes of the received PAM signal. Often, the received PAM signal is filtered before data bit timing is recovered. Such filtering would soften (decrease the slope of) the transition leaving less energy for differentiation. Filtering, variable amplitudes of the data bits, and noise all combine to produce a jitter which degrades performance of this prior art device.
Another prior art device uses the frame synchronization pulses only to recover data bit timing. This technique is unsatisfactory for many applications because it is slow, requires very low loop bandwidths and has a very narrow acquisition range.